Luminescent material and method of manufacture



Patented Jan. 13, 1942 LUM'INESGENT MATERIAL AND lVIETHOD OF MANUFACTUREJames N. Bowtell, Sudbury, Henry G.- Jenkins, Pinner, and Alfred H.McKeag, Nortl'rWembley, England, assignors to General Electric Gompany,a corporation of New York No Drawing. Application April 18, 1940. SerialNo. 330,372 In Great Britain April 19, 1939 3 Claims.

Our invention relates to light producing means comprising in combinationasource adapted, when in operation, to emit either the mercury or a raregas and particularly the neon spectrum, and luminescent material exposedto the radiation of the said source during its operation and therebyexcited to luminescence by it. More particularly, our invention relatesto a luminescent material for such light producing means, and. to amethod of preparing such luminescent material.

Calcium tungstate is well known as the luminescent material in means ofthis type. More recently magnesium tungstate (as disclosed in BritishPatent No. 469,732) and zinc tungstate (as disclosed in British PatentNo. 469,913) have replaced calcium tungstate for many purposes,especially when the source of exciting radiations emits the mercury andnot the neon spectrum, because the luminescent efiiciency of suchtungstates is higher than that of calcium tungstate. More recentlystill, magnesium-cadmium tungstate has been proposed, as disclosed inco-pending U. S. application Serial No. 235,914, filed October 19, 1938.Calciiun tungstate differs from these three substances in emitting lightof a much more saturated blue.

Cadmium tungstate is known to be luminescent under suitable excitingagencies, but so far as We are aware, it has never been used in anycommercial light source of the type specified, or ofiered for commercialsale as a luminescent material. The reason is probably that it has beenfound diificult to prepare it by the method usually employed in themanufacture of other luminescent tungstates so that it has a luminescentefiiciency comparable even with that of calcium tungstate.

One object of our invention is to provide luminescent cadmium tungstatehaving a luminescent efficiency at least as great as that of calciumtungstate and not very greatly less than that of magnesium tungstate.

Another object of our invention is to provide a method of preparingcadmium tungstate having such improved luminescent efiiciency. The colorof the luminescent light emitted by cadmium tungstate prepared by ourimproved method resembles that of magnesium tungstate rather than thatof calcium tungstate.

According to the invention, in light producing means of the typespecified, the luminescent material is, or comprises a substantialproportion of, luminescent cadmium tungstate.

In View of the said co-pending application, it

is to be understood that in the preceding statement the words have theirconventional meaning. When luminescent material is said to compriseluminescent material X, it is to be understood that the presence ofanymaterial other than luminescent material X isnot associated- WithX so asto change-the luminescent properties of X. Thus; the material describedin the said co-pending application does not comprise either magnesium orcadmium tungstate, because in that material the two tungstates areassociated so that each affects the luminescent properties of the other.

One method of preparing luminescent cadmium tungstate according to ourinvention will now be described by way of example.

198 gm. of sodium tungstate of AnalaR (analytical reagent) grade isdissolved in 1 litre (1000 ml.) of hot distilled water. The resultingsolution is called solution A. While the volume of water is slightlyincreased due to the addition of the sodium tungstate, for the purposesof computation, the volume of the resulting solution may be assumed toequal approximately 1 litre or 1000 ml.

154 gm. of cadmium sulphate of AnalaR (analytical reagent) grade isdissolved in 1 litre of hot distilled water. The resulting solution iscalled solution B. Here again, for purposes of computation, the volumeof solution B may be considered to equal approximately 1 litre or 1000ml.

10 m1. (1. e. approximately 1%) of solution A is added to the whole ofsolution B. The resulting precipitate is filtered off 'and discarded.

The remainder (i. e. approximately 99%) of solution A is then added tothe filtrate. The precipitate is filtered off, washed 6 times with hotdistilled water and dried at 180 C. It is then heated at 875 C. for onehour in air, washed again 6 times with hot distilled water, and dried at180 C. in air. The resulting material is then the improved luminescentcadmium. tungstate comprising our invention.

It is to be observed that the deliberate introduction of an activator,such as lead, is not necessary; but the use of activators is notexcluded. It is well-known that contradictory statements have been madeabout the necessity or desirability of activators in tungstates. It islikely that the same difiiculty will arise in determining theadvisiblity of employing activators in cadmium tungstate.

As has been said, luminescent cadmium tungstate resembles magnesiumtungstate in its luminescent properties rather than calcium tungstate,and is therefore, in general, better adapted to replace the formerrather than the latter. It may be used inside a low pressure mercuryvapour lamp, or outside a high-pressure mercury-vapour lamp so long asits envelope is sulficiently transparent to short-wave radiation. It mayalso be used inside a neon discharge tube.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A method of producing luminescent cadmium tungstate which comprisespreparing a solution A of pure sodium tungstate dissolved in distilledwater in the proportions of 198 grams of sodium tungstate to 1 litre ofwater, preparing a substantially equivalent amount of a solution B ofpure cadmium sulphate dissolved in distilled water in the proportions of154 grams of cadmium sulphate to 1 litre of water, adding approximately1% of solution A to the whole of solution B and filtering off theprecipitate, adding to the resulting filtrate the remainder of solutionA and again filtering ofi the precipitate, then washing the latterprecipitate in distilled water, drying it, and. heating it atapproximately 875 C. for one hour in air.

2. A method of preparing luminescent cadmium tungstate which comprisesmixing an aqueous solution of alkali tungstate with a substantiallyequivalent amount of an aqueous solution of cadmium sulphate, addingapproximately one per cent of the alkali tungstate solution to thecadmium sulphate solution and filtering off the precipitate, adding tothe resulting filtrate the remainder of the alkali tungstate solutionand again filtering off the precipitate, washing the latter precipitate,drying it, and then heating it at a temperature of approximately 875 C.to form the luminescent cadmium tungstate.

3. A method of preparing luminescent cadmium' tungstate which comprisesmixing an aqueous solution of sodium tungstate with a substantiallyequivalent amount of an aqueous solution of cadmium sulphate, addingapproximately one per cent of the sodium tungstate solution to thecadmium sulphate solution and filtering oil the precipitate, adding tothe resulting filtrate the remainder of the sodium tungstate solutionand again filtering off the precipitate, washing the latter precipitate,drying it, and then heating it at a temperature of approximately 875 C.to form the luminescent cadmium tungstate.

JAMES N. BOWTELL. HENRY G. JENKINS. ALFRED H. MCKEAG.

